The invention relates to a push-pull output stage having an input terminal, an output terminal, a first supply terminal, and a second supply terminal, which stage comprises:
A first transistor of a first conductivity type, connected in common-collector arrangement, whose collector is coupled to the first supply terminal, whose emitter is coupled to the output terminal and whose base is coupled to the input terminal;
a second transistor of the first conductivity type, connected in common-emitter arrangement, whose emitter is coupled to the second supply terminal, PA1 a third transistor of a second conductivity type opposite to the first conductivity type and connected in common-base arrangement, PA1 a first signal path, which includes the emitter-collector path of the first transistor and which serves for driving the second transistor as a function of the drive of the first transistor, and PA1 a first semiconductor junction with a first and a second electrode, which semiconductor junction is included in the collector main-current path of the second transistor and whose first electrode is coupled to the output terminal and the emitter of the first transistor, in such a way that viewed from the output terminal the forward direction of the first semiconductor junction is opposed to the forward direction of the base-emitter junction of the first transistor, and whose second electrode is coupled to the collector of the second transistor. PA1 a measuring circuit at least comprising the base-emitter junction of a fourth transistor of the first conductivity type, which measuring circuit is included between the base of the first transistor and the second electrode of the first semiconductor junction, for generating a drive current, which is derived from the voltage between the base of the first transistor and the second electrode of the first semi-conductor junction and for applying said drive current to the base of the second transistor, in phase opposition to the input signal via the first signal path, PA1 a first impedance in the first signal path, of which one terminal is coupled to the emitter of the third transistor, PA1 first means for bypassing a part of the first signal path, which part comprises the first impedance and the emitter-collector path of the third transistor, with a suitable second impedance which at least transfers signals of comparatively high frequencies, so that in the case of poor high-frequency properties of the third transistor, the push-pull output stage still performs satisfactorily at high frequencies. PA1 a sixth transistor of the first conductivity type, whose base is coupled to the input terminal, whose emitter is coupled to the base of the fourth transistor via a first junction point, and whose collector is coupled to the emitter of the fifth transistor and the base of the first transistor, PA1 a second impedance circuit, suitably a current-source circuit, which is included between the base of the fifth transistor and the first supply terminal, PA1 a third impedance circuit, suitably a current-source circuit, which is included between the first junction point and the second supply terminal, and PA1 a third semiconductor junction, which is connected in series with the second semiconductor junction between the base of the fifth transistor and the first junction point, the second and the third semiconductor junction being included in forward direction, while furthermore PA1 the collector of the fifth transistor is coupled to the first supply terminal and the emitter of the fourth transistor is coupled to the second electrode of the second semiconductor junction. PA1 because the collector of the sixth transistor carries substantially the same signal as its base, the influence of the "Miller" capacitance is substantially reduced, so that considerably better high frequency properties are obtained when driving the push-pull output stage from a high-ohmic signal source; PA1 and that the main current of the sixth transistor also passes through the main current path of the fifth transistor, so that one current-source circuit may be dispensed with.
A push-pull output stage of the type mentioned in the opening paragraph is known from: "Electronics Letters" Vol. 10, No. 15 of July 25, 1974, pages 317, 318 and 319.
In order to enable the second transistor in this circuit to be driven in a suitable manner, the collector circuit of the first transistor includes a diode in forward direction. Via a third transistor of a conductivity type opposite to that of the first and the second transistor, which third transistor is connected in common-base arrangement, the voltage across said diode, which is a measure of the current through the first transistor, is converted into a drive current which is in phase opposition to the input signal, which drive current is applied to the base of the second transistor. As a result of this, the current through the second transistor will be in phase opposition to that through the first transistor.
In an integrated push-pull output stage of this type, the first and the second transistors will generally be vertical npn transistors, while it is necessary to employ a horizontal pnp transistor for the third transistor, which as is known has poor high-frequency properties. In order to mitigate the influence of said poor high-frequency properties in this circuit arrangement, it is advisable to bypass the emitter-collector path of the third transistor with a series connection of a resistor and a capacitor. However, the signal voltage on the emitter of the third transistor is insufficient to drive the second transistor via the series connection of the resistor and capacitor, so that the band-width and the drive range at high frequencies are very limited.
The asymmetry of the measuring circuit in the collector circuit of the first transistor and the drive of the second transistor via a resistor in parallel with its base-emitter junction, causes a substantial temperature dependence of the quiescent current setting and a non-linear transfer. Furthermore, the input impedance of this circuit arrangement is highly dependent on the value of the input signal.